This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to German Application No. 100 53 818.5 filed on Oct. 30, 2000, the entire content of which is incorporated herein by reference.
The invention generally relates to an electrical steering system. More particularly, the present invention pertains to an electrical steering system for a vehicle with an electrically supplied steering motor arrangement, which is connected with an inverter arrangement, whose output voltage is influenced by a computer arrangement, and with a sensor arrangement.
A known electrical steering system is disclosed in DE 196 25 350 A1. Such steering system is preferably used with fork lifts and other self-propelled working machines, particularly with those operating in closed rooms, like for example warehouses. In this case, there is no longer a mechanical connection between the steering handwheel or a comparable control device and the steered wheel(s). Thus, in a manner of speaking, the driver or the operator has, in case of a failure of the steering system, no possibility of intervening in the steering behavior of the vehicle. It is therefore usually a requirement that each fault is immediately recognized and that the vehicle is immediately stopped, when a fault occurs. However, also in this connection dangerous situations may occur, as a vehicle with a large weight will always require a certain braking distance. The higher the weight of the vehicle, the longer the braking distance.
The invention is based on the task of equipping a steering system in such a way that it remains fully steerable during braking, at the lowest cost and effort possible.
With a steering system as mentioned in the introduction, this task is solved in that the steering motor arrangement has a redundant steering motor, which is connected with the computer arrangement via two separately run control circuits, which have separate electric supplies, the computer arrangement also being redundant.
With this embodiment a redundant steering system can be achieved at a reasonable cost and effort. It would be relatively easy to design a fault-tolerant system by doubling all elements. However, this solution is relatively expensive. The invention now foresees that not all elements are doubled, but that elements, which contribute considerably to the costs, are made redundant per se. Thus, for the major part of the steering system a two-circuit design is available, which also functions, when a fault occurs in one of the two circuits. The remaining elements, which are used in common in both circuits, are so fault tolerant, however, that they continue to work also when a fault has occurred. The result is a steering system, which is optimized with regard to costs, and which is nonetheless able to ensure full steering properties until the vehicle stops.
It is preferred that all control circuits have a common desired value specification device. For example, a steering handwheel can act as desired value specification device. It is assumed that a broken steering handwheel column is a very unlikely fault. Therefore, a protection against this fault has not been made. However, the operation is drastically simplified, when compared with systems in which it must be ensured that the driver activates both control circuits with independent operating elements.
Preferably, each control circuit has an independent current supply device having its own monitoring system. Such a current supply device can, for example, be a battery arrangement. The fact that each current supply device supplies own components, namely those of its own control circuit, ensures that the functioning of the steering system is also ensured when one current supply device fails. Each current supply device has its own monitoring, for example a voltage measuring, which reports back to the computer arrangement. When one current supply arrangement fails, the computer arrangement can then initiate the stopping procedure.
Preferably, each control circuit has an inverter, which is connected with the steering motor. Each inverter works with, for example, voltage modulations, typically with a pulse width modulation, and receives the corresponding modulation signals from the computer arrangement. Each inverter is supplied with voltage from its own current supply device, namely that of the belonging control circuit. The failure of an inverter is therefore uncritical for the steering ability of the vehicle on a whole.
Preferably, the steering motor has one winding arrangement for each control circuit. It can, for example, be made so that one stator is provided, which has a set of windings for each control circuit, which sets of windings act upon a common rotor. However, each set of windings can also have its own stator, each stator cooperating with a rotor, both rotors then being mounted on a common shaft. This gives the advantage that during faultless operation no negative mutual interferences can appear, as only one steering motor arrangement acts mechanically on the steered wheel(s). In case of a fault, however, one half of the steering motor can, in a manner of speaking, still provide the force required to steer the vehicle until it stands completely still.
It is particularly preferred that during failure of at least one inverter, each inverter is overloaded with the supply of the motor. This means that the inverters can be dimensioned in such a way that only together they are able to provide the electrical output required to control the steering motor arrangement. In case of a fault, one inverter is then overloaded. As, however, this overload only lasts until the vehicle stops, this overload can be accepted.
Preferably, each control circuit has a steering handwheel sensor. Thus, the fault redundancy increase can start already at a relatively early stage and it is ensured that all movements of the steering handwheel or a similar device can be registered several times.
It is preferred that the sensors are arranged in a common sensor unit. This gives the advantage that the coordination of the output signals of the sensors with each other is more easily realized. A sensor arrangement of this kind is disclosed in, for example, DE 31 45 162 A1.
Preferably, each control circuit has a midpoint sensor for the steering motor arrangement. In faultlessly working multi-phase systems, all phases work symmetrically. The midpoint of a star connection, which can also be called xe2x80x9cstar pointxe2x80x9d, is always located on a specified potential, for example, the zero potential. Electrical faults in the windings, a short-circuit phasexe2x80x94phase or phase-ground or other faults will cause an asymmetry, which can be recorded by means of a measuring in the midpoint or the star point of the motor. Also a fault in an inverter will cause an asymmetry in the motor, and thus be recordable in the midpoint.
Preferably, the computer arrangement has at least two computers, of which one is made as a master computer and the other as a slave computer. This has the advantage that the slave computer can be made in a less expensive way. In a manner of speaking, the master computer assumes the xe2x80x9cleading rolexe2x80x9d. In this connection, xe2x80x9ccomputerxe2x80x9d means the totality of an arrangement, which is able to process incoming signals and convert them to output signals for the steering and, if required, for the driving of the vehicle, that is, having particularly a processor with input and output interfaces and a memory device for a program to be performed. Of course, it is not necessary for such a computer to have a keyboard and a screen or replaceable data carriers.
Preferably, the slave computer is made as a computer with reduced control capacity, which only serves the purpose of maintaining the steering ability. This means that during normal operation, the master computer can control the steering function, whereas, due to its reduced capacity, the production of the slave computer can be substantially cheaper.
Preferably, the two computers monitor each other. This improves the reliability of recording faults occurring in the computers.
It is also preferred that on occurrence of a fault, the faultless computer turns off the failing computer. Thus, a faulty computer is prevented from having further influence on the system.
Preferably, during faultless operation one computer controls all inverters, and in case of a fault, the failing computer hands over its control to another computer. This has the advantage that synchronization problems in connection with the control of the inverters will not occur. As all inverters are controlled in the same way, also the steering motor arrangement can be operated without interferences. Also in case of faults this situation does not change, as the control of the inverters is handed over completely from one computer to the other.
Preferably, the computer arrangement sends a modulation signal and an activation signal to each inverter, the activation signal for each inverter being individually variable. This simplifies the disconnection of an inverter when a fault occurs in this inverter or in the motor connected with it. An interference with the modulation signals is thus not possible, on the contrary, it is sufficient to change the activation signal for the inverter in question, for example to turn off this inverter.